The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with sheet or "fingerprint" type gradient coil designs for magnetic resonance imaging apparatus and will be described with particular reference thereto. However, it will be appreciated that the present invention will also find application in conjunction with the generation of magnetic fields and gradients for other applications.
Heretofore, magnetic resonance imagers have included a superconducting magnet which generated a temporally constant primary magnetic field through a central bore. A series of annular gradient magnetic field coils for generating x, y, and z-gradient magnetic field gradients were mounted to an interior of the bore. An annular radio frequency coil was commonly positioned in the interior of the gradient coils for transmitting radio frequency signals into and receiving radio frequency magnetic resonance signals from a subject in the bore. Current pulses were applied to the gradient and radio frequency coils to generate a series of RF and gradient field pulses of conventional magnetic resonance imaging sequences.
Various coil constructions have been used for generating the x, y, and z magnetic field gradients. One type of gradient coil includes a flexible, dielectric backing layer to which a sheet of copper or other conductive foil material has been laminated. A coil pattern was defined by cutting, milling, or etching the conductive sheet in a generally spiral-like pattern. The conductive sheet remaining between the spiral cut provided a generally spiral or fingerprint-like current path. See, for example, U.S. Pat. No. 5,177,442 to Roemer or U.S. Pat. No. 4,840,700 to Edelstein. The conductive pattern represents an approximation of a continuous current density vector J. Mathematically, the current density J=curl S, where S is a stream function representing contours of constant integrated current density. A number of turns N is selected and the coil is patterned into N+1 contours, each of constant S, which differ in magnitude by an amount .DELTA.S=S.sub.max /N. Hypothetical contours offset from these by .DELTA.S/2 represent a desired pattern for placing filamentary wires to approximate J.
The contour lines generated by this method determine the machining pattern or cut lines. The cut lines were formed by removing a constant width of the conductor material producing an electrical discontinuity or gap which defines the turn pattern. Typically, the conductive sheet is copper and has a thickness between 1 and 2 mm. The cut lines are typically 2 mm. wide, i.e. about equal to the sheet thickness.
Because the current density function J varies across the sheet, this process of removing a constant width cut line to define the coil pattern results in conductors of varying width. The current can spread in the wide areas and becomes concentrated in the narrow conductor areas such that the selected current density J is better met.
One of the difficulties which arises is that in regions of high current density, only narrow segments of conductor remain between the cut lines, e.g. a width of about 4 mm. This reduction in conductor width from a larger nominal width, e.g. 6 mm, is disadvantageous. The power loss, I.sup.2 R, increases markedly with the increase in current density in the narrow regions. Moreover, the heat generation rate .rho.J.sup.2 (where .rho. is electrical resistivity), also increases markedly in the high current density regions. Reducing the conductor width from 6 mm. to 4 mm. increases the power loss by a factor of about 1.5 and increases the heat generation rate by a factor of about 2.25. The localized coil heating is a significant disadvantage in the prior art.
Another disadvantage of the prior art is that the wider portions of the conductor support eddy currents. The gradient coil commonly includes a layered assembly of x, y, and z-coils. All three are mounted in very close proximity to one another and are driven in a pulsed manner. When a coil is driven with a current pulse, the resultant magnetic field induces eddy currents in neighboring conductors wherever possible. These eddy currents reduce the driving magnetic field and have an associated decay time. Because the neighboring coils are also fingerprint coils, the available current paths for eddy currents have a non-constant spatial distribution. Thus, the eddy current patterns are spatially dissimilar from the driving field. Both the extended duration of the time decay and the spatial dissimilarity are disadvantageous in magnetic resonance applications. In addition, the electric field generated by the differences in voltages between adjacent coils results in capacitive coupling. Charging this capacitance also has an associated time constant. It is usually desirable to minimize the time constants and currents associated with this process as well.
Wire wound gradient coils also have disadvantages. The wire width is limited by the minimum contour separation distance and the wire height/thickness is limited by radial build constraints. The smallest cross-section is used to wind the entire spiral coil. This results in a coil of higher resistance and higher total heat generation.
In accordance with the present invention, a new and improved gradient coil configuration is provided which overcomes the above-referenced problems and others.